Interpretive Summary: Toxoplasma gondii is a single-celled parasite of all warm-blooded hosts worldwide. It causes mental retardation and loss of vision in children, and abortion in livestock. Cats are the main reservoir of T. gondii because they are the only hosts that can excrete the resistant stage (oocyst) of the parasite in the feces. Humans become infected by eating undercooked meat from infected animals and food and water contaminated with oocysts. In the present study, authors report prevalence of Toxoplasma in sheep in Mexico. The results will be of interest to biologists, parasitologists, and public health workers.

Technical Abstract:
The seroprevalence of Toxoplasma gondii infection in sheep in northern Mexico is largely unknown. Antibodies to T. gondii were determined in serum samples from 511 sheep from 8 farms in Durango State, Mexico using the modified agglutination test (MAT). Sheep were raised in 3 geographical regions, i.e., mountainous (n=68), semi-desert (n=132), and valley (n=311). Overall, T. gondii antibodies were found in 77 (15.1%) of 511 sheep, with MAT titers of 1:25 in 27, 1:50 in 10, 1:100 in 11, 1:200 in 11, 1:400 in 8, 1:800 in 3, 1:1,600 in 4, and 1:3,200 in 3. The seroprevalence of T. gondii infection increased significantly with age, indicating post-natal transmission. In contrast, gender, breed, flock size, and geographic region did not significantly influence the seroprevalence. Seropositive sheep were found in 7 of 8 farms sampled. This is the first report of T. gondii infection in sheep in Durango State, Mexico. Results indicate that infected sheep are likely an important source of T. gondii infection in humans in Durango State.